Catch that DV!

Was talking to someone today about the hardware specs needed to capture DV footage (straight from a DV camera's firewire output through a firewire interface to a hard drive). Since the last parenthetical statement revels the need for a DV camera and a firewire interface, and there's not a big range of differences when talking firewire interfaces - the hardware specs in question were - you guessed it - the mouse. (Err...hard drive - although get a wireless optical mouse, you'll love it. And while your getting that mouse, get one for me too.)

So here's a timlytle.net DV capture primer. Why? So when someone asks me, "What do I need to capture the DV stuff from my DV camera thingy?" - I can say, "Just go to my site, and read the timlytle.net DV Capture Primer." Will I ever be asked that question again? Probably not.

Capturing DV takes a DV camera, a computer, and a firewire interface. (Yes, you could use a capture device that captures DV from an analogue source and converts it to DV, but that'll probably output the DV to firewire - just like the camera.) We'll ignore all the computer components except for the hard drive (although, I wouldn't try to capture DV on a 485 DX2-50MHz, even if it had a SATA RAID Array [okay, I would try]), use common sense - you'll need a decent processor and some RAM.

The DV format stores or sends data at 3.6MB/sec (that's megabytes per second). The Firewire standard is now capable of 800 Mb/sec (that's megabits per second) - it used to be 400 Mb/sec. Since there's at least 8 bits per byte ("At least?" you ask - well, I'm thinking there may be some parity bits floating around somewhere in the Firewire standard) the DV format storage (or transmission) rate would be 3.6 * 8 megabits/second or 28 Mb/sec. Seems pretty easy for Firewire to do - and it is; however, it may not be that simple for your hard drive. When DV is being transfered, that rate (3.6 MB/sec) must be sustained. That means your hard drive must be capable of writing a continuous 3.6 MB/sec.

Is your hard drive capable of doing that? Your drive's 'write speed' is probably much grater than 3.8MB/sec - but for capture the speed can never drop below 3.6Mb/sec. In english? Well, you drive may be able to write fast enough most of the time, but if it 'stutters' once and the write speed drops below 3.6MB/sec frames get dropped. That's generally a bad thing.

Why would your drive stutter? Could be something else requested some data from the drive, and that interrupted the write process, or it could be that while writing the file, it ran into another file and had to find space somewhere else (that would be fragmentation).

What can be done to prevent this? Well, you could defragment your hard drive (that's a good thing to do anyway), that would help with the latter problem. You can make sure no other applications are running, or use a second drive for the capture (that would help with the first problem). Having a large 'write cache' on you drive would help with both of the above problems.

Here's an example - once I ran a drive speed test developed specifically to determine if a drive could capture DV on two identical IDE drives. One was the system drive (the main drive for the computer, the one the operating system ran off of) and the other was on it's own IDE interface. The test showed that the independent drive was able to capture DV, and the system drive was not. Once again, the drives were identical.

Once upon a time IDE drives weren't fast enough for capturing DV - you had to use SCSI. Now an IDE drive can be fast enough, if it's a fast drive to begin with (7200 RPM), if it's not fragmented, and it helps if it's isolate. A second drive is better, and a second drive as the only device on an IDE channel is best.

Just a few hours ago I tested capturing DV to a system drive (7200, 2MB buffer, Ultra ATA133) on a relatively new (< 1 year) DV editing computer. The capture worked fine with no dropped frames, for about 20 seconds. So does that mean that all of the above was ramblings of no value. Yeah - pretty much. No, wait - no it wasn't. The preceding concepts still apply - would the afford mentioned system be able to reliable capture long clips? I don't really know - I didn't test it for more than 30 seconds. How well would a 'normal' computer capture to the system drive (the tested computer is use for very little other than DV work)? But the bottom line is this - you can capture using pretty much any (7200) drive around - but keep in mind the preceding paragraph if you start dropping frames.

Of course using SATA drives, or putting drives is RAID would remove most (if not all) speed problems with capturing DV.

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